How to control image transparency like photoshop
you can add another "mixshader", connect the output of the current "mixshader" to the new. 

Afterwards you can connect the "transparent shader" to the top of the new "mixshader".

The factor can be a "color ramp" that is dependent on Z of the object (in fact you can use whatever factor here to control another transparency).

Check the printscreen of the proposed node setup: